/*
    * Copyright (C) 2008-2009, Google Inc.
    * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
    * under the terms of the Eclipse Distribution License v1.0 which
    * accompanies this distribution, is reproduced below, and is
    * available at http://www.eclipse.org/org/documents/edl-v10.php
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or
   * without modification, are permitted provided that the following
   * conditions are met:
   *
   * - Redistributions of source code must retain the above copyright
   *   notice, this list of conditions and the following disclaimer.
   *
   * - Redistributions in binary form must reproduce the above
   *   copyright notice, this list of conditions and the following
   *   disclaimer in the documentation and/or other materials provided
   *   with the distribution.
   *
   * - Neither the name of the Eclipse Foundation, Inc. nor the
   *   names of its contributors may be used to endorse or promote
   *   products derived from this software without specific prior
   *   written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
   * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
   * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
   * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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  package org.eclipse.jgit.treewalk;
  
  import java.io.IOException;
  
  import org.eclipse.jgit.errors.CorruptObjectException;
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.errors.StopWalkException;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.lib.FileMode;
  import org.eclipse.jgit.lib.MutableObjectId;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.ObjectReader;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.revwalk.RevTree;
  import org.eclipse.jgit.treewalk.filter.PathFilter;
  import org.eclipse.jgit.treewalk.filter.TreeFilter;
  import org.eclipse.jgit.util.RawParseUtils;
  
  /**
   * Walks one or more {@link AbstractTreeIterator}s in parallel.
   * <p>
   * This class can perform n-way differences across as many trees as necessary.
   * <p>
   * Each tree added must have the same root as existing trees in the walk.
   * <p>
   * A TreeWalk instance can only be used once to generate results. Running a
   * second time requires creating a new TreeWalk instance, or invoking
   * {@link #reset()} and adding new trees before starting again. Resetting an
   * existing instance may be faster for some applications as some internal
   * buffers may be recycled.
   * <p>
   * TreeWalk instances are not thread-safe. Applications must either restrict
   * usage of a TreeWalk instance to a single thread, or implement their own
   * synchronization at a higher level.
   * <p>
   * Multiple simultaneous TreeWalk instances per {@link Repository} are
   * permitted, even from concurrent threads.
   */
  public class TreeWalk implements AutoCloseable {
  	private static final AbstractTreeIterator[] NO_TREES = {};
  
  	/**
  	 * Open a tree walk and filter to exactly one path.
  	 * <p>
  	 * The returned tree walk is already positioned on the requested path, so
  	 * the caller should not need to invoke {@link #next()} unless they are
  	 * looking for a possible directory/file name conflict.
  	 *
  	 * @param reader
  	 *            the reader the walker will obtain tree data from.
  	 * @param path
  	 *            single path to advance the tree walk instance into.
  	 * @param trees
 	 *            one or more trees to walk through, all with the same root.
 	 * @return a new tree walk configured for exactly this one path; null if no
 	 *         path was found in any of the trees.
 	 * @throws IOException
 	 *             reading a pack file or loose object failed.
 	 * @throws CorruptObjectException
 	 *             an tree object could not be read as its data stream did not
 	 *             appear to be a tree, or could not be inflated.
 	 * @throws IncorrectObjectTypeException
 	 *             an object we expected to be a tree was not a tree.
 	 * @throws MissingObjectException
 	 *             a tree object was not found.
 	 */
 	public static TreeWalk forPath(final ObjectReader reader, final String path,
 			final AnyObjectId... trees) throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		TreeWalk tw = new TreeWalk(reader);
 		PathFilter f = PathFilter.create(path);
 		tw.setFilter(f);
 		tw.reset(trees);
 		tw.setRecursive(false);
 
 		while (tw.next()) {
 			if (f.isDone(tw)) {
 				return tw;
 			} else if (tw.isSubtree()) {
 				tw.enterSubtree();
 			}
 		}
 		return null;
 	}
 
 	/**
 	 * Open a tree walk and filter to exactly one path.
 	 * <p>
 	 * The returned tree walk is already positioned on the requested path, so
 	 * the caller should not need to invoke {@link #next()} unless they are
 	 * looking for a possible directory/file name conflict.
 	 *
 	 * @param db
 	 *            repository to read tree object data from.
 	 * @param path
 	 *            single path to advance the tree walk instance into.
 	 * @param trees
 	 *            one or more trees to walk through, all with the same root.
 	 * @return a new tree walk configured for exactly this one path; null if no
 	 *         path was found in any of the trees.
 	 * @throws IOException
 	 *             reading a pack file or loose object failed.
 	 * @throws CorruptObjectException
 	 *             an tree object could not be read as its data stream did not
 	 *             appear to be a tree, or could not be inflated.
 	 * @throws IncorrectObjectTypeException
 	 *             an object we expected to be a tree was not a tree.
 	 * @throws MissingObjectException
 	 *             a tree object was not found.
 	 */
 	public static TreeWalk forPath(final Repository db, final String path,
 			final AnyObjectId... trees) throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		try (ObjectReader reader = db.newObjectReader()) {
 			return forPath(reader, path, trees);
 		}
 	}
 
 	/**
 	 * Open a tree walk and filter to exactly one path.
 	 * <p>
 	 * The returned tree walk is already positioned on the requested path, so
 	 * the caller should not need to invoke {@link #next()} unless they are
 	 * looking for a possible directory/file name conflict.
 	 *
 	 * @param db
 	 *            repository to read tree object data from.
 	 * @param path
 	 *            single path to advance the tree walk instance into.
 	 * @param tree
 	 *            the single tree to walk through.
 	 * @return a new tree walk configured for exactly this one path; null if no
 	 *         path was found in any of the trees.
 	 * @throws IOException
 	 *             reading a pack file or loose object failed.
 	 * @throws CorruptObjectException
 	 *             an tree object could not be read as its data stream did not
 	 *             appear to be a tree, or could not be inflated.
 	 * @throws IncorrectObjectTypeException
 	 *             an object we expected to be a tree was not a tree.
 	 * @throws MissingObjectException
 	 *             a tree object was not found.
 	 */
 	public static TreeWalk forPath(final Repository db, final String path,
 			final RevTree tree) throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		return forPath(db, path, new ObjectId[] { tree });
 	}
 
 	private final ObjectReader reader;
 
 	private final boolean closeReader;
 
 	private final MutableObjectId idBuffer = new MutableObjectId();
 
 	private TreeFilter filter;
 
 	AbstractTreeIterator[] trees;
 
 	private boolean recursive;
 
 	private boolean postOrderTraversal;
 
 	private int depth;
 
 	private boolean advance;
 
 	private boolean postChildren;
 
 	AbstractTreeIterator currentHead;
 
 	/**
 	 * Create a new tree walker for a given repository.
 	 *
 	 * @param repo
 	 *            the repository the walker will obtain data from. An
 	 *            ObjectReader will be created by the walker, and will be closed
 	 *            when the walker is closed.
 	 */
 	public TreeWalk(final Repository repo) {
 		this(repo.newObjectReader(), true);
 	}
 
 	/**
 	 * Create a new tree walker for a given repository.
 	 *
 	 * @param or
 	 *            the reader the walker will obtain tree data from. The reader
 	 *            is not closed when the walker is closed.
 	 */
 	public TreeWalk(final ObjectReader or) {
 		this(or, false);
 	}
 
 	private TreeWalk(final ObjectReader or, final boolean closeReader) {
 		reader = or;
 		filter = TreeFilter.ALL;
 		trees = NO_TREES;
 		this.closeReader = closeReader;
 	}
 
 	/** @return the reader this walker is using to load objects. */
 	public ObjectReader getObjectReader() {
 		return reader;
 	}
 
 	/**
 	 * Release any resources used by this walker's reader.
 	 * <p>
 	 * A walker that has been released can be used again, but may need to be
 	 * released after the subsequent usage.
 	 *
 	 * @since 4.0
 	 */
 	@Override
 	public void close() {
 		if (closeReader) {
 			reader.close();
 		}
 	}
 
 	/**
 	 * Get the currently configured filter.
 	 *
 	 * @return the current filter. Never null as a filter is always needed.
 	 */
 	public TreeFilter getFilter() {
 		return filter;
 	}
 
 	/**
 	 * Set the tree entry filter for this walker.
 	 * <p>
 	 * Multiple filters may be combined by constructing an arbitrary tree of
 	 * <code>AndTreeFilter</code> or <code>OrTreeFilter</code> instances to
 	 * describe the boolean expression required by the application. Custom
 	 * filter implementations may also be constructed by applications.
 	 * <p>
 	 * Note that filters are not thread-safe and may not be shared by concurrent
 	 * TreeWalk instances. Every TreeWalk must be supplied its own unique
 	 * filter, unless the filter implementation specifically states it is (and
 	 * always will be) thread-safe. Callers may use {@link TreeFilter#clone()}
 	 * to create a unique filter tree for this TreeWalk instance.
 	 *
 	 * @param newFilter
 	 *            the new filter. If null the special {@link TreeFilter#ALL}
 	 *            filter will be used instead, as it matches every entry.
 	 * @see org.eclipse.jgit.treewalk.filter.AndTreeFilter
 	 * @see org.eclipse.jgit.treewalk.filter.OrTreeFilter
 	 */
 	public void setFilter(final TreeFilter newFilter) {
 		filter = newFilter != null ? newFilter : TreeFilter.ALL;
 	}
 
 	/**
 	 * Is this walker automatically entering into subtrees?
 	 * <p>
 	 * If the walker is recursive then the caller will not see a subtree node
 	 * and instead will only receive file nodes in all relevant subtrees.
 	 *
 	 * @return true if automatically entering subtrees is enabled.
 	 */
 	public boolean isRecursive() {
 		return recursive;
 	}
 
 	/**
 	 * Set the walker to enter (or not enter) subtrees automatically.
 	 * <p>
 	 * If recursive mode is enabled the walker will hide subtree nodes from the
 	 * calling application and will produce only file level nodes. If a tree
 	 * (directory) is deleted then all of the file level nodes will appear to be
 	 * deleted, recursively, through as many levels as necessary to account for
 	 * all entries.
 	 *
 	 * @param b
 	 *            true to skip subtree nodes and only obtain files nodes.
 	 */
 	public void setRecursive(final boolean b) {
 		recursive = b;
 	}
 
 	/**
 	 * Does this walker return a tree entry after it exits the subtree?
 	 * <p>
 	 * If post order traversal is enabled then the walker will return a subtree
 	 * after it has returned the last entry within that subtree. This may cause
 	 * a subtree to be seen by the application twice if {@link #isRecursive()}
 	 * is false, as the application will see it once, call
 	 * {@link #enterSubtree()}, and then see it again as it leaves the subtree.
 	 * <p>
 	 * If an application does not enable {@link #isRecursive()} and it does not
 	 * call {@link #enterSubtree()} then the tree is returned only once as none
 	 * of the children were processed.
 	 *
 	 * @return true if subtrees are returned after entries within the subtree.
 	 */
 	public boolean isPostOrderTraversal() {
 		return postOrderTraversal;
 	}
 
 	/**
 	 * Set the walker to return trees after their children.
 	 *
 	 * @param b
 	 *            true to get trees after their children.
 	 * @see #isPostOrderTraversal()
 	 */
 	public void setPostOrderTraversal(final boolean b) {
 		postOrderTraversal = b;
 	}
 
 	/** Reset this walker so new tree iterators can be added to it. */
 	public void reset() {
 		trees = NO_TREES;
 		advance = false;
 		depth = 0;
 	}
 
 	/**
 	 * Reset this walker to run over a single existing tree.
 	 *
 	 * @param id
 	 *            the tree we need to parse. The walker will execute over this
 	 *            single tree if the reset is successful.
 	 * @throws MissingObjectException
 	 *             the given tree object does not exist in this repository.
 	 * @throws IncorrectObjectTypeException
 	 *             the given object id does not denote a tree, but instead names
 	 *             some other non-tree type of object. Note that commits are not
 	 *             trees, even if they are sometimes called a "tree-ish".
 	 * @throws CorruptObjectException
 	 *             the object claimed to be a tree, but its contents did not
 	 *             appear to be a tree. The repository may have data corruption.
 	 * @throws IOException
 	 *             a loose object or pack file could not be read.
 	 */
 	public void reset(final AnyObjectId id) throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		if (trees.length == 1) {
 			AbstractTreeIterator o = trees[0];
 			while (o.parent != null)
 				o = o.parent;
 			if (o instanceof CanonicalTreeParser) {
 				o.matches = null;
 				o.matchShift = 0;
 				((CanonicalTreeParser) o).reset(reader, id);
 				trees[0] = o;
 			} else {
 				trees[0] = parserFor(id);
 			}
 		} else {
 			trees = new AbstractTreeIterator[] { parserFor(id) };
 		}
 
 		advance = false;
 		depth = 0;
 	}
 
 	/**
 	 * Reset this walker to run over a set of existing trees.
 	 *
 	 * @param ids
 	 *            the trees we need to parse. The walker will execute over this
 	 *            many parallel trees if the reset is successful.
 	 * @throws MissingObjectException
 	 *             the given tree object does not exist in this repository.
 	 * @throws IncorrectObjectTypeException
 	 *             the given object id does not denote a tree, but instead names
 	 *             some other non-tree type of object. Note that commits are not
 	 *             trees, even if they are sometimes called a "tree-ish".
 	 * @throws CorruptObjectException
 	 *             the object claimed to be a tree, but its contents did not
 	 *             appear to be a tree. The repository may have data corruption.
 	 * @throws IOException
 	 *             a loose object or pack file could not be read.
 	 */
 	public void reset(final AnyObjectId... ids) throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		final int oldLen = trees.length;
 		final int newLen = ids.length;
 		final AbstractTreeIterator[] r = newLen == oldLen ? trees
 				: new AbstractTreeIterator[newLen];
 		for (int i = 0; i < newLen; i++) {
 			AbstractTreeIterator o;
 
 			if (i < oldLen) {
 				o = trees[i];
 				while (o.parent != null)
 					o = o.parent;
 				if (o instanceof CanonicalTreeParser && o.pathOffset == 0) {
 					o.matches = null;
 					o.matchShift = 0;
 					((CanonicalTreeParser) o).reset(reader, ids[i]);
 					r[i] = o;
 					continue;
 				}
 			}
 
 			o = parserFor(ids[i]);
 			r[i] = o;
 		}
 
 		trees = r;
 		advance = false;
 		depth = 0;
 	}
 
 	/**
 	 * Add an already existing tree object for walking.
 	 * <p>
 	 * The position of this tree is returned to the caller, in case the caller
 	 * has lost track of the order they added the trees into the walker.
 	 * <p>
 	 * The tree must have the same root as existing trees in the walk.
 	 *
 	 * @param id
 	 *            identity of the tree object the caller wants walked.
 	 * @return position of this tree within the walker.
 	 * @throws MissingObjectException
 	 *             the given tree object does not exist in this repository.
 	 * @throws IncorrectObjectTypeException
 	 *             the given object id does not denote a tree, but instead names
 	 *             some other non-tree type of object. Note that commits are not
 	 *             trees, even if they are sometimes called a "tree-ish".
 	 * @throws CorruptObjectException
 	 *             the object claimed to be a tree, but its contents did not
 	 *             appear to be a tree. The repository may have data corruption.
 	 * @throws IOException
 	 *             a loose object or pack file could not be read.
 	 */
 	public int addTree(final AnyObjectId id) throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		return addTree(parserFor(id));
 	}
 
 	/**
 	 * Add an already created tree iterator for walking.
 	 * <p>
 	 * The position of this tree is returned to the caller, in case the caller
 	 * has lost track of the order they added the trees into the walker.
 	 * <p>
 	 * The tree which the iterator operates on must have the same root as
 	 * existing trees in the walk.
 	 *
 	 * @param p
 	 *            an iterator to walk over. The iterator should be new, with no
 	 *            parent, and should still be positioned before the first entry.
 	 *            The tree which the iterator operates on must have the same root
 	 *            as other trees in the walk.
 	 *
 	 * @return position of this tree within the walker.
 	 * @throws CorruptObjectException
 	 *             the iterator was unable to obtain its first entry, due to
 	 *             possible data corruption within the backing data store.
 	 */
 	public int addTree(final AbstractTreeIterator p)
 			throws CorruptObjectException {
 		final int n = trees.length;
 		final AbstractTreeIterator[] newTrees = new AbstractTreeIterator[n + 1];
 
 		System.arraycopy(trees, 0, newTrees, 0, n);
 		newTrees[n] = p;
 		p.matches = null;
 		p.matchShift = 0;
 
 		trees = newTrees;
 		return n;
 	}
 
 	/**
 	 * Get the number of trees known to this walker.
 	 *
 	 * @return the total number of trees this walker is iterating over.
 	 */
 	public int getTreeCount() {
 		return trees.length;
 	}
 
 	/**
 	 * Advance this walker to the next relevant entry.
 	 *
 	 * @return true if there is an entry available; false if all entries have
 	 *         been walked and the walk of this set of tree iterators is over.
 	 * @throws MissingObjectException
 	 *             {@link #isRecursive()} was enabled, a subtree was found, but
 	 *             the subtree object does not exist in this repository. The
 	 *             repository may be missing objects.
 	 * @throws IncorrectObjectTypeException
 	 *             {@link #isRecursive()} was enabled, a subtree was found, and
 	 *             the subtree id does not denote a tree, but instead names some
 	 *             other non-tree type of object. The repository may have data
 	 *             corruption.
 	 * @throws CorruptObjectException
 	 *             the contents of a tree did not appear to be a tree. The
 	 *             repository may have data corruption.
 	 * @throws IOException
 	 *             a loose object or pack file could not be read.
 	 */
 	public boolean next() throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		try {
 			if (advance) {
 				advance = false;
 				postChildren = false;
 				popEntriesEqual();
 			}
 
 			for (;;) {
 				final AbstractTreeIterator t = min();
 				if (t.eof()) {
 					if (depth > 0) {
 						exitSubtree();
 						if (postOrderTraversal) {
 							advance = true;
 							postChildren = true;
 							return true;
 						}
 						popEntriesEqual();
 						continue;
 					}
 					return false;
 				}
 
 				currentHead = t;
 				if (!filter.include(this)) {
 					skipEntriesEqual();
 					continue;
 				}
 
 				if (recursive && FileMode.TREE.equals(t.mode)) {
 					enterSubtree();
 					continue;
 				}
 
 				advance = true;
 				return true;
 			}
 		} catch (StopWalkException stop) {
 			for (final AbstractTreeIterator t : trees)
 				t.stopWalk();
 			return false;
 		}
 	}
 
 	/**
 	 * Obtain the tree iterator for the current entry.
 	 * <p>
 	 * Entering into (or exiting out of) a subtree causes the current tree
 	 * iterator instance to be changed for the nth tree. This allows the tree
 	 * iterators to manage only one list of items, with the diving handled by
 	 * recursive trees.
 	 *
 	 * @param <T>
 	 *            type of the tree iterator expected by the caller.
 	 * @param nth
 	 *            tree to obtain the current iterator of.
 	 * @param clazz
 	 *            type of the tree iterator expected by the caller.
 	 * @return r the current iterator of the requested type; null if the tree
 	 *         has no entry to match the current path.
 	 */
 	@SuppressWarnings("unchecked")
 	public <T extends AbstractTreeIterator> T getTree(final int nth,
 			final Class<T> clazz) {
 		final AbstractTreeIterator t = trees[nth];
 		return t.matches == currentHead ? (T) t : null;
 	}
 
 	/**
 	 * Obtain the raw {@link FileMode} bits for the current entry.
 	 * <p>
 	 * Every added tree supplies mode bits, even if the tree does not contain
 	 * the current entry. In the latter case {@link FileMode#MISSING}'s mode
 	 * bits (0) are returned.
 	 *
 	 * @param nth
 	 *            tree to obtain the mode bits from.
 	 * @return mode bits for the current entry of the nth tree.
 	 * @see FileMode#fromBits(int)
 	 */
 	public int getRawMode(final int nth) {
 		final AbstractTreeIterator t = trees[nth];
 		return t.matches == currentHead ? t.mode : 0;
 	}
 
 	/**
 	 * Obtain the {@link FileMode} for the current entry.
 	 * <p>
 	 * Every added tree supplies a mode, even if the tree does not contain the
 	 * current entry. In the latter case {@link FileMode#MISSING} is returned.
 	 *
 	 * @param nth
 	 *            tree to obtain the mode from.
 	 * @return mode for the current entry of the nth tree.
 	 */
 	public FileMode getFileMode(final int nth) {
 		return FileMode.fromBits(getRawMode(nth));
 	}
 
 	/**
 	 * Obtain the ObjectId for the current entry.
 	 * <p>
 	 * Using this method to compare ObjectId values between trees of this walker
 	 * is very inefficient. Applications should try to use
 	 * {@link #idEqual(int, int)} or {@link #getObjectId(MutableObjectId, int)}
 	 * whenever possible.
 	 * <p>
 	 * Every tree supplies an object id, even if the tree does not contain the
 	 * current entry. In the latter case {@link ObjectId#zeroId()} is returned.
 	 *
 	 * @param nth
 	 *            tree to obtain the object identifier from.
 	 * @return object identifier for the current tree entry.
 	 * @see #getObjectId(MutableObjectId, int)
 	 * @see #idEqual(int, int)
 	 */
 	public ObjectId getObjectId(final int nth) {
 		final AbstractTreeIterator t = trees[nth];
 		return t.matches == currentHead ? t.getEntryObjectId() : ObjectId
 				.zeroId();
 	}
 
 	/**
 	 * Obtain the ObjectId for the current entry.
 	 * <p>
 	 * Every tree supplies an object id, even if the tree does not contain the
 	 * current entry. In the latter case {@link ObjectId#zeroId()} is supplied.
 	 * <p>
 	 * Applications should try to use {@link #idEqual(int, int)} when possible
 	 * as it avoids conversion overheads.
 	 *
 	 * @param out
 	 *            buffer to copy the object id into.
 	 * @param nth
 	 *            tree to obtain the object identifier from.
 	 * @see #idEqual(int, int)
 	 */
 	public void getObjectId(final MutableObjectId out, final int nth) {
 		final AbstractTreeIterator t = trees[nth];
 		if (t.matches == currentHead)
 			t.getEntryObjectId(out);
 		else
 			out.clear();
 	}
 
 	/**
 	 * Compare two tree's current ObjectId values for equality.
 	 *
 	 * @param nthA
 	 *            first tree to compare the object id from.
 	 * @param nthB
 	 *            second tree to compare the object id from.
 	 * @return result of
 	 *         <code>getObjectId(nthA).equals(getObjectId(nthB))</code>.
 	 * @see #getObjectId(int)
 	 */
 	public boolean idEqual(final int nthA, final int nthB) {
 		final AbstractTreeIterator ch = currentHead;
 		final AbstractTreeIterator a = trees[nthA];
 		final AbstractTreeIterator b = trees[nthB];
 		if (a.matches != ch && b.matches != ch) {
 			// If neither tree matches the current path node then neither
 			// tree has this entry. In such case the ObjectId is zero(),
 			// and zero() is always equal to zero().
 			//
 			return true;
 		}
 		if (!a.hasId() || !b.hasId())
 			return false;
 		if (a.matches == ch && b.matches == ch)
 			return a.idEqual(b);
 		return false;
 	}
 
 	/**
 	 * Get the current entry's name within its parent tree.
 	 * <p>
 	 * This method is not very efficient and is primarily meant for debugging
 	 * and final output generation. Applications should try to avoid calling it,
 	 * and if invoked do so only once per interesting entry, where the name is
 	 * absolutely required for correct function.
 	 *
 	 * @return name of the current entry within the parent tree (or directory).
 	 *         The name never includes a '/'.
 	 */
 	public String getNameString() {
 		final AbstractTreeIterator t = currentHead;
 		final int off = t.pathOffset;
 		final int end = t.pathLen;
 		return RawParseUtils.decode(Constants.CHARSET, t.path, off, end);
 	}
 
 	/**
 	 * Get the current entry's complete path.
 	 * <p>
 	 * This method is not very efficient and is primarily meant for debugging
 	 * and final output generation. Applications should try to avoid calling it,
 	 * and if invoked do so only once per interesting entry, where the name is
 	 * absolutely required for correct function.
 	 *
 	 * @return complete path of the current entry, from the root of the
 	 *         repository. If the current entry is in a subtree there will be at
 	 *         least one '/' in the returned string.
 	 */
 	public String getPathString() {
 		return pathOf(currentHead);
 	}
 
 	/**
 	 * Get the current entry's complete path as a UTF-8 byte array.
 	 *
 	 * @return complete path of the current entry, from the root of the
 	 *         repository. If the current entry is in a subtree there will be at
 	 *         least one '/' in the returned string.
 	 */
 	public byte[] getRawPath() {
 		final AbstractTreeIterator t = currentHead;
 		final int n = t.pathLen;
 		final byte[] r = new byte[n];
 		System.arraycopy(t.path, 0, r, 0, n);
 		return r;
 	}
 
 	/**
 	 * @return The path length of the current entry.
 	 */
 	public int getPathLength() {
 		return currentHead.pathLen;
 	}
 
 	/**
 	 * Test if the supplied path matches the current entry's path.
 	 * <p>
 	 * This method tests that the supplied path is exactly equal to the current
 	 * entry, or is one of its parent directories. It is faster to use this
 	 * method then to use {@link #getPathString()} to first create a String
 	 * object, then test <code>startsWith</code> or some other type of string
 	 * match function.
 	 *
 	 * @param p
 	 *            path buffer to test. Callers should ensure the path does not
 	 *            end with '/' prior to invocation.
 	 * @param pLen
 	 *            number of bytes from <code>buf</code> to test.
 	 * @return &lt; 0 if p is before the current path; 0 if p matches the current
 	 *         path; 1 if the current path is past p and p will never match
 	 *         again on this tree walk.
 	 */
 	public int isPathPrefix(final byte[] p, final int pLen) {
 		final AbstractTreeIterator t = currentHead;
 		final byte[] c = t.path;
 		final int cLen = t.pathLen;
 		int ci;
 
 		for (ci = 0; ci < cLen && ci < pLen; ci++) {
 			final int c_value = (c[ci] & 0xff) - (p[ci] & 0xff);
 			if (c_value != 0)
 				return c_value;
 		}
 
 		if (ci < cLen) {
 			// Ran out of pattern but we still had current data.
 			// If c[ci] == '/' then pattern matches the subtree.
 			// Otherwise we cannot be certain so we return -1.
 			//
 			return c[ci] == '/' ? 0 : -1;
 		}
 
 		if (ci < pLen) {
 			// Ran out of current, but we still have pattern data.
 			// If p[ci] == '/' then pattern matches this subtree,
 			// otherwise we cannot be certain so we return -1.
 			//
 			return p[ci] == '/' ? 0 : -1;
 		}
 
 		// Both strings are identical.
 		//
 		return 0;
 	}
 
 	/**
 	 * Test if the supplied path matches (being suffix of) the current entry's
 	 * path.
 	 * <p>
 	 * This method tests that the supplied path is exactly equal to the current
 	 * entry, or is relative to one of entry's parent directories. It is faster
 	 * to use this method then to use {@link #getPathString()} to first create
 	 * a String object, then test <code>endsWith</code> or some other type of
 	 * string match function.
 	 *
 	 * @param p
 	 *            path buffer to test.
 	 * @param pLen
 	 *            number of bytes from <code>buf</code> to test.
 	 * @return true if p is suffix of the current path;
 	 *         false if otherwise
 	 */
 	public boolean isPathSuffix(final byte[] p, final int pLen) {
 		final AbstractTreeIterator t = currentHead;
 		final byte[] c = t.path;
 		final int cLen = t.pathLen;
 
 		for (int i = 1; i <= pLen; i++) {
 			// Pattern longer than current path
 			if (i > cLen)
 				return false;
 			// Current path doesn't match pattern
 			if (c[cLen - i] != p[pLen - i])
 				return false;
 		}
 
 		// Whole pattern tested -> matches
 		return true;
 	}
 
 	/**
 	 * Get the current subtree depth of this walker.
 	 *
 	 * @return the current subtree depth of this walker.
 	 */
 	public int getDepth() {
 		return depth;
 	}
 
 	/**
 	 * Is the current entry a subtree?
 	 * <p>
 	 * This method is faster then testing the raw mode bits of all trees to see
 	 * if any of them are a subtree. If at least one is a subtree then this
 	 * method will return true.
 	 *
 	 * @return true if {@link #enterSubtree()} will work on the current node.
 	 */
 	public boolean isSubtree() {
 		return FileMode.TREE.equals(currentHead.mode);
 	}
 
 	/**
 	 * Is the current entry a subtree returned after its children?
 	 *
 	 * @return true if the current node is a tree that has been returned after
 	 *         its children were already processed.
 	 * @see #isPostOrderTraversal()
 	 */
 	public boolean isPostChildren() {
 		return postChildren && isSubtree();
 	}
 
 	/**
 	 * Enter into the current subtree.
 	 * <p>
 	 * If the current entry is a subtree this method arranges for its children
 	 * to be returned before the next sibling following the subtree is returned.
 	 *
 	 * @throws MissingObjectException
 	 *             a subtree was found, but the subtree object does not exist in
 	 *             this repository. The repository may be missing objects.
 	 * @throws IncorrectObjectTypeException
 	 *             a subtree was found, and the subtree id does not denote a
 	 *             tree, but instead names some other non-tree type of object.
 	 *             The repository may have data corruption.
 	 * @throws CorruptObjectException
 	 *             the contents of a tree did not appear to be a tree. The
 	 *             repository may have data corruption.
 	 * @throws IOException
 	 *             a loose object or pack file could not be read.
 	 */
 	public void enterSubtree() throws MissingObjectException,
 			IncorrectObjectTypeException, CorruptObjectException, IOException {
 		final AbstractTreeIterator ch = currentHead;
 		final AbstractTreeIterator[] tmp = new AbstractTreeIterator[trees.length];
 		for (int i = 0; i < trees.length; i++) {
 			final AbstractTreeIterator t = trees[i];
 			final AbstractTreeIterator n;
 			if (t.matches == ch && !t.eof() && FileMode.TREE.equals(t.mode))
 				n = t.createSubtreeIterator(reader, idBuffer);
 			else
 				n = t.createEmptyTreeIterator();
 			tmp[i] = n;
 		}
 		depth++;
 		advance = false;
 		System.arraycopy(tmp, 0, trees, 0, trees.length);
 	}
 
 	@SuppressWarnings("unused")
 	AbstractTreeIterator min() throws CorruptObjectException {
 		int i = 0;
 		AbstractTreeIterator minRef = trees[i];
 		while (minRef.eof() && ++i < trees.length)
 			minRef = trees[i];
 		if (minRef.eof())
 			return minRef;
 
 		minRef.matches = minRef;
 		while (++i < trees.length) {
 			final AbstractTreeIterator t = trees[i];
 			if (t.eof())
 				continue;
 			final int cmp = t.pathCompare(minRef);
 			if (cmp < 0) {
 				t.matches = t;
 				minRef = t;
 			} else if (cmp == 0) {
 				t.matches = minRef;
 			}
 		}
 
 		return minRef;
 	}
 
 	void popEntriesEqual() throws CorruptObjectException {
 		final AbstractTreeIterator ch = currentHead;
 		for (int i = 0; i < trees.length; i++) {
 			final AbstractTreeIterator t = trees[i];
 			if (t.matches == ch) {
 				t.next(1);
 				t.matches = null;
 			}
 		}
 	}
 
 	void skipEntriesEqual() throws CorruptObjectException {
 		final AbstractTreeIterator ch = currentHead;
 		for (int i = 0; i < trees.length; i++) {
 			final AbstractTreeIterator t = trees[i];
 			if (t.matches == ch) {
 				t.skip();
 				t.matches = null;
 			}
 		}
 	}
 
 	private void exitSubtree() {
 		depth--;
 		for (int i = 0; i < trees.length; i++)
 			trees[i] = trees[i].parent;
 
 		AbstractTreeIterator minRef = null;
 		for (final AbstractTreeIterator t : trees) {
 			if (t.matches != t)
 				continue;
 			if (minRef == null || t.pathCompare(minRef) < 0)
 				minRef = t;
 		}
 		currentHead = minRef;
 	}
 
 	private CanonicalTreeParser parserFor(final AnyObjectId id)
 			throws IncorrectObjectTypeException, IOException {
 		final CanonicalTreeParser p = new CanonicalTreeParser();
 		p.reset(reader, id);
 		return p;
 	}
 
 	static String pathOf(final AbstractTreeIterator t) {
 		return RawParseUtils.decode(Constants.CHARSET, t.path, 0, t.pathLen);
 	}
 
 	static String pathOf(final byte[] buf, int pos, int end) {
 		return RawParseUtils.decode(Constants.CHARSET, buf, pos, end);
 	}
 }